This invention relates to an optical information recording and reproducing apparatus, or in particular to an optical information recording and reproducing apparatus high in recording density and having a high super resolution effect.
With the recent progress of the information society using optical telecommunication, the construction of a telecommunication system capable of high-speed communication of information high in recording density has been required. An optical information recording and reproducing apparatus capable of accumulating the optical information of high recording density is an indispensable optical device for developing the high-speed optical telecommunication of high recording density. Further, with the digitization of video information such as TV images and the increased image quality such as high definition, it is of urgent necessity to develop an information recording and reproducing apparatus of high recording density capable of long-time recording while maintaining a high image quality.
Currently, a DVD having a capacity of 4.7 GB on each side finds wide applications as an optical information recording medium for handling high-density dynamic images such as computer and video information. The practical application of this DVD as a rewritable recording and reproducing medium as well as a read-only ROM (DVD-ROM) with information written directly in the substrate has been promoted. The development of these optical information recording media is aimed at a high recording density, and the laser light having a shorter wavelength of 650 nm than the laser (780 nm) for the CD is used as a means to achieve the high density information recording. For applications to the computer graphics and the digital high-definition images involving the information of large recording capacity, however, the recording density four to five times higher is required. In order to meet this requirement, an optical disk using a blue semiconductor laser still shorter in wavelength (405 nm) and having the recording density of 23.3 GB on one side has been developed and found practical applications.
As a technique to further increase the recording density of the optical disk, the development of a multilayer recording method, a multi-valued recording method and a super resolution recording method is under way. Of these next-generation methods to achieve a high recording density, the super resolution recording method is one of the most promising techniques.
In the super resolution recording method, the waist of the laser beam radiated on the recording surface is reduced using the laser focusing function or the masking function of the super resolution layer. This is one of the recording methods of high density recording realized by the reversible change in the optical constants (refractive index (n) and the extinction coefficient (k)) of the super resolution layer formed in a multilayer structure such as the recording layer, the protection layer or the reflection layer of the optical disk. The super resolution layer, upon radiation of the read/write laser thereon, is excited by the temperature rise or the absorption of photons of light. As long as the laser is radiated, therefore, the refractive index and the extinction coefficient are changed reversibly, while the original state is restored upon extinction of the laser. In the optical disk, a recording portion and a non-recording portion are determined for reproduction by the amount of the laser light returned to the pickup after being radiated and reflected on the optical disk. Due to this reversible change of the optical constants in the super resolution layer, the area of the light returned to the pickup can be reduced more than the normal radiation area of the laser light. Specifically, the resolution can be improved by reducing the readable area using the optical masking effect.
The extinction coefficient (k) is a quantity proportional to the light absorption coefficient of a material, and assumes a larger value the larger the absorption coefficient of the material. The two constants including the refractive index (n) and the absorption coefficient (k) are collectively called the optical constants.
In the prior art, as described in JP-A-10-340482, for example, a thin film material of cobalt oxide has been used as the super resolution layer. The large change in refractive index and the super resolution effect of this thin film can produce an optical disk of high recording density.
In the optical information recording and reproducing apparatus currently available, the optical information is reproduced by radiating the continuous wave (CW) light or the reproducing laser light superposed with high frequencies of about 400 MHz. In the case where the the complex refractive index of the super resolution layer is changed and the power of the reproducing light is increased until the super resolution effect is obtained, therefore, heat is accumulated on the optical information recording medium by the radiation of laser light, and a broad heat distribution occurs in the laser beam spot, thereby posing the problem that a super resolution mask high in contrast cannot be formed. Further, the heat accumulation degradate the film or the recording pit on the medium, resulting in the degradation of the repetitive reproducing operation characteristic.
A reproducing method with pulse light for improving the super resolution effect by avoiding the heat accumulation in the medium and steepening the temperature gradient in the beam spot is described, for example, in JP-A-10-40547.
In an application of the pulse reproducing method as described in JP-A-10-40547, a high response of the material is essential. Also, the beam spot in super resolution state, which is a superposition of a beam spot in ground state and a beam spot in super resolution state, is affected by the beam spot in ground state, and a satisfactory super resolution mask is difficult to obtain.